Unfired or unburned magnesite-carbon refractory furnace brick or plates (hereafter "shapes") are known in the art for lining metallurgical vessels or for slide gate plates and contain, for example, 5 to 30 percent carbon by weight. These shapes are formed with a carbonaceous binder and baked at about 400.degree. F. to cure the binder. Shapes of this type are disclosed, for example, in U.S. Pat. Nos. 5,007,615; 4,306,030 and 4,957,887. U.S. Pat. No. 5,007,615 discloses an unfired magnesite-carbon slide gate plate containing at least 5% by weight carbon and at least 74% by weight magnesite. This patent teaches that it is only possible to have up to 3% carbon in a fired magnesite plate. U.S. Pat. No. 4,306,030 teaches the addition of aluminum powder and/or magnesium powder and silicon powder to an unfired magnesite-carbon brick. U.S. Pat. No. 4,957,887 teaches the use of high-purity magnesite and high-purity graphite in a magnesite-carbon brick containing aluminum metal, magnesium metal or mixtures thereof. This patent teaches that it is undesirable to include silica in the form of impurities in the magnesite or the graphite or as separate additions to the brick.
The magnesite-carbon refractory material described in these references should be distinguished from fired, carbon impregnated refractory shapes. These later products are formed from batches that contain no carbon, are fired in oxidizing atmospheres to form a sintered ceramic oxide bond, and are then impregnated with tar to provide them with a carbon content up to about five percent.
The unfired magnesite-carbon brick have desirable slag resistance properties due to the high carbon content. The principal use of such brick has been in furnace linings.
In modern steel mills, steel is teemed from ladles and tundishes through sliding gate valves at the bottom of these vessels. The sliding gate valves comprise two or three plates having holes therein that can be brought into and out of registry by the relative movement of the plates. The plates of the sliding gate valves must have good erosion resistance as well as excellent mechanical properties at elevated temperatures. The plates and associated nozzles are made of various refractory compositions. Hot modulus of rupture at 2700.degree. F. (1480.degree. C.) is a good measure of the mechanical strength required by plates in slide gates. A strong, cohesive and erosion resistant grain boundary is required to maintain a cohesive refractory structure at these elevated temperatures.
Recent development of aggressive grades of steel has necessitated the creation of different types of ceramic compositions capable of controlling the flow of the molten steel under severe casting conditions. Magnesia products have been developed for high temperature (ladle treatment) or aggressive steels and slag casting conditions. Magnesia could also have potential use for basic or alkaline steel compositions containing calcium and/or manganese. Heretofore it has not been possible to manufacture satisfactory magnesia shapes for slide gate applications having the necessary abrasion resistance and thermal shock resistance due to the failure to provide a suitable bonding structure for magnesia grains.
Up to this time, it has not been possible to create an acceptable bonding structure for magnesia grains. Prior carbon bonded magnesia ceramic compositions have proven to be a weak material, possessing low physical properties, so the inherent chemical advantage of the magnesia has heretofore not been fully realized.